Frequency tunable semiconductor diode lasers provide versatile optical tools for telecommunications, metrology, spectroscopy and other uses. Many such tunable lasers use a diffraction grating with a movable reflector to select a desired wavelength from the beam diffracted by the grating. A diode gain medium is employed that has an antireflection (AR) coating on one facet thereof. Light emitted from the AR coated facet is diffracted by a grating and directed to a movable reflector, which feeds light back to the grating and gain medium. Rotation of the reflector selects the wavelength diffracted by the grating and allows the laser to be tuned to a desired output wavelength. Translational motion of the reflector is frequently employed in conjunction with the rotational motion to couple the cavity optical path length to the selected wavelength and provide mode-hop free tuning. Grating-tuned external cavity lasers are typically arranged in the Littman-Metcalf configuration with a xe2x80x9cfolded cavityxe2x80x9d, which permits compact-sized external cavity laser devices suitable for many commercial uses.
The optical output of grating-tuned external cavity lasers of this sort may be collected as the light emitted from a rear, partially reflective facet of the gain medium, or as the grating reflection of light directly from the gain medium. This provides a relatively high output power, but includes xe2x80x9cnoisexe2x80x9d in the form of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) from the gain medium. One approach to providing a spectrally xe2x80x9ccleanxe2x80x9d output from grating-tuned external cavity lasers has been to simply insert a beam coupler directly into the laser cavity between the grating and gain medium. A partially reflective surface on the beam coupler directs a portion of the light returning from the grating outside the cavity. This partially reflected light is at the selected wavelength and has been spatially separated from the propagation direction of the spontaneous emission light by the grating. This spectrally clean output may then be coupled into a fiber for use in applications requiring high spectral purity.
This relatively simple approach to providing a spectrally pure output beam has some important drawbacks. One of the attractive features of folded cavity lasers is the small or compact size that is possible for commercial lasers. Directing optical output outside of the folded cavity results in a substantial increase in the overall size and complexity of the external cavity laser device. Further, the introduction of a beam coupler into the laser cavity results in a significant intracavity optical loss. The insertion of a beam coupler into the laser cavity always results in the extra optical loss from the opposite reflection off the partially reflective surface of the beam coupler from the spectrally cleaned light that is collected and use. The spectral cleansing provided by beam couplers thus is obtained with a corresponding sacrifice in laser output power.
There is a need for an external cavity laser apparatus that provides suppression of spontaneous emission light from laser output, that is simple and compact in design, which provides high laser output power, and which collects loss components associated with spectral cleaning as usable laser outputs. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.
The invention provides a laser apparatus and method with compact cavity design that provides suppression of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) light with minimal intracavity loss. The apparatus comprises a gain medium emitting a light beam along an optical path, a tuning element positioned in the optical path and configured feed back light of a selected wavelength to the gain medium and configured to define a first output beam directed along a first output path, a partial reflector located in the optical path and positioned to create a second output beam directed along a second output path substantially parallel to the first output path; and having a spontaneous emission component that is spatially separated from the selected wavelength.
By way of example, and not of limitation, the apparatus may further comprise an optical fiber positioned with respect to the second output path such that light at the selected wavelength is selectively received by or coupled into the optical fiber. A reflector may be positioned in the optical path after the tuning element to define an external laser cavity with a facet of the gain medium. The reflector may be movable with respect to the tuning element to define the selected wavelength. The tuning element may comprise a grating, an etalon, an interference filter, or other optical element or capable of providing wavelength selection.
In certain embodiments, the apparatus may comprise a beam coupler positioned in the optical path, with the partial reflector located on a facet of the beam coupler. The beam coupler may further comprise an antireflection coating on one or more facets that are opposite from the partial reflector on the beam coupler. In certain embodiments, the beam coupler may be configured to define a third output beam traveling a third output path that is substantially parallel to the first and second output paths.
By way of further example, the external cavity of the apparatus, in some embodiments, has a folded external cavity design and comprises a reflective rear facet on the gain medium and a reflector positioned in the optical path after the tuning element, wherein the reflector and a rear facet of the gain medium define the external laser cavity. The tuning element comprises a tuning grating capable of selecting a specific wavelength for output from the external cavity. The external cavity is folded with respect to the tuning grating, and the reflector is movable with respect to the grating to provide wavelength selection. A first output beam is reflected from the tuning grating along a first output path, and contains noise associated with source spontaneous emission (SSE) and/or amplified spontaneous emission (ASE) associated with current pumping of the gain medium.
A beam coupler, which may comprise a simple optical flat with a partially reflective surface and an anti-reflection-coated surface, is positioned in the optical path between the gain medium and the tuning grating. The beam coupler is positioned in the optical path to receive light diffracted from the tuning grating and to reflect a portion of this light out of the external cavity as a second output beam along a second output path that is substantially or approximately parallel to the first output path. The second output beam comprises a portion of the light diffracted from the tuning-grating toward the gain medium, and which is intercepted or picked off by the beam coupler before the light can be fed back to or otherwise return to the gain medium. The beam coupler may, in certain embodiments, have a wedge or prism configuration.
In the second output beam, the spontaneous emission light generated in the gain medium has been spatially separated from light at the selected wavelength by operation of the tuning grating, and has dispersion characteristics for generating a low-noise output. The second output beam is coupled to an optical fiber positioned and configured to selectively receive the light at the tuned or selected wavelength, and to selectively exclude light associated with spontaneous emission. Since the second output path is substantially or approximately parallel to the first output path, coupling of the both the first and second output beams into fibers can be achieved without sacrificing the overall compact size of the external cavity laser apparatus. This arrangement of the output paths allows for ease of alignment and detection of either the first or second output beam from the laser apparatus.
In certain embodiments, the beam coupler may be configured to provide a third output beam along a third output path. The third output beam takes advantage of the reflection of the direct output beam from the gain medium off the partial reflector in the beam coupler, which would otherwise be uncollected and result in optical loss. The apparatus may be configured so that the third output path is substantially parallel to the first and second output paths. The third output beam is of relatively high power output power compared to the second output beam, but contains SSE or ASE noise components that have not been spatially separated from the selected wavelength.
In still other embodiments, a portion of an output beam or beams may be directed to an external diffraction grating or optical filtering devices. The incidence of the beam on the diffraction grating will spread or spatially separate the selected wavelength from the noise or spontaneous emission background and direct the selected wavelength to coupling optics. A portion of the beam directed to the coupling optics is picked off and directed to a quadcell detector, a linear array detector, or other detector. Output from the detector is provided to a controller that is operatively coupled to the grating and is configured to tune the grating in a manner that optimizes coupling of the output beam into the coupling optics. The orientation of the fiber optical axis, together with the spatial filtering or separation provided by the grating, provide for filtering of spontaneous emission components from the light coupled into the fiber. The controller tunes the tuning element according to feedback from the detector to keep the grating tuned or oriented for optimum coupling of the output beam into the optical fiber.
The methods of the invention comprise, in general terms, emitting a light beam from a gain medium along an optical path, feeding light of a selected wavelength back to the gain medium by a tuning element positioned in the optical path, forming or creating a first output beam that is directed along a first output path, creating or forming a second output beam directed along a second output path parallel to the first output path, and spatially separating spontaneous emission light from light of the selected wavelength in the second output beam.
The subject methods may further comprise coupling the second output beam into an optical fiber positioned to selectively receive the selected wavelength and exclude spontaneous emission light. Creating the second output path may comprise positioning a partial mirror in the optical path between the tuning element and gain medium. The methods may additionally comprise defining an external laser cavity by placing a reflector in the optical path after the tuning element, with the external cavity defined by the reflector and a facet of the gain medium. Positioning the partial mirror in the optical path may comprise positioning a beam coupler in the optical path, with the partial reflector located on a facet of the beam coupler. The methods may still further comprise creating a third output beam by the beam coupler, with the third output beam traveling along a third output path parallel to the first and second output paths.
The invention provides a tunable external cavity with a compact or folded design that produces a continuously tunable output with extremely low noise from source spontaneous emission (SSE) and amplified spontaneous emission (ASE). The apparatus and methods of the invention can be utilized in the testing, measurement and evaluation of optical systems, WDM, DWDM, EDFA, fiber network, optical passive components, metrology, spectroscopy, industrial process monitoring, optical analytical instrumentation and Raman spectroscopy. The invention further provides a laser apparatus that generates multiple-beam outputs of frequency tunable coherent light sources with both a low noise, SSE and ASE-suppressed beam and one or more high output power beams without ASE and SSE suppression. The invention further provides for use of cavity loss as effective laser output and allows for multiple laser light output sources to be accessed from a single unit. With the output path of the light beam with reduced SSE or ASE positioned to be substantially parallel to the primary output beam, the laser apparatus can take advantage of the compact size offered by the inventive cavity configurations.